We thank the National Institutes of Health (GM46059) for financial support of this project and for a postdoctoral fellowship to A.T.P. (F32GM093532). The Varian 300 MHz and Bruker 400 MHz NMR spectrometers used in this work were supported by grants from the National Science Foundation (ACHE-9808061 and DBI-9729592) and the National Institutes of Health (1S10RR13886-01), respectively.